The present invention relates to a short cell multiplexing apparatus for routing a short cell by using an asynchronous transfer mode (ATM) switching system.
Upon carrying out communication by an ATM, data is loaded in an ATM cell which is a unit of data switching. The ATM cell is transmitted through an ATM connection setting previously. The ATM cell is, when transmitted within the ATM network, allocated with one destination information (virtual path identifier/virtual channel identifier (VPI/VCI)) per connection. The VPI/VCI is loaded in the header of the ATM cell. The ATM cell is transmitted through the ATM connection corresponding to the VPI/VCI.
In the technical field relating to mobile communication, data is converted into a compressed low-bit rate data format for its transmission so that a transmission band may effectively be used. If the low-speed bit rate information is loaded into the payload of a standard ATM cell, much time is required so that the payload of one ATM cell is filled with data. For this reason, there is a fear that there occurs a delay of data transmission and a drop of communication quality.
Under the above circumstances, a multiplexing transfer system called AAL Type 2 serving as a system capable of transmitting low-bit rate information with less delay has been investigated while focusing on ITU-T. FIG. 18 is diagram showing an ATM cell of AAL Type 2 format (hereinafter referred to as AAL Type 2 cell) having multiplexed short cells in the payload. FIG. 19 is a table showing header information stored in the AAL Type 2 cell and the short cell in FIG. 18. The AAL Type 2 format has recently been recommended as the ATM cell for transmitting a plurality of short cells.
As showing in FIG. 18, the header of the AAL Type 2 cell has each field of the standard cell header (5 byte) and each field of OSF, SN, P (1 byte). Consequently, the AAL Type 2 cell differs from the standard cell in payload length (47 bytes).
The short cell consists of a short cell header and a short cell payload. A short cell connection identifier (CID) for identifying short cell connection and a length indicator (LI) for indicating the payload length of the short cell are loaded in the short cell header. On the other hand, the low-bit rate information mentioned above is loaded in the short cell payload. Hereinafter, xe2x80x9cAAL Type 2 cellxe2x80x9d is prescribed to mean an AAL Type 2 cell storing a plurality of short cells.
However, upon transmitting the AAL Type 2 cell by using the ATM connection as described above, there occurred the following problems. Namely, a plurality of short cells having different CIDs are multiplexed in the payload of the AAL Type 2 cell. Therefore, each short cell is not transmitted to a desired destination unless an ATM switching apparatus carries out switching per short cell. However, conventional ATM switching apparatuses do not comprise a function for switching each short cell in the ATM cell for every short cell.
In this case, if the ATM switching apparatus has a function of carrying out the switching per short cell by processing to the AAL type 2 cell, it is preferable that a constitution of the ATM switching apparatus may be simple. For example, when a AAL type 2 cell is inputted to a ATM switching apparatus, if the ATM switching apparatus extracts a plurality of short cells, generates standard ATM cells storing a short cell among the extracted short cells (hereafter, this standard ATM cell is called xe2x80x9cpartial fill cellxe2x80x9d: see in FIG. 18), and carries out switching per partial fill cell, the ATM switching apparatus is able to realize switching per short cell. An apparatus converting partial fill cells into a AAL type 2 cell is called a short cell multiplexing apparatus. An apparatus converting a AAL type 2 cell into partial fill cell is called a short cell demultiplexing apparatus.
The short cell multiplexing apparatus has to have a function that when partial fill cells converted a AAL type 2 cell, new destination information is given the AAL type 2 cell. The new destination information has to uniquely identify per call (connection).
An ATM switching apparatus has a table memorizing an input side VPI/VCI and an output side VPI/VCI corresponding to the input side VPI/VCI. When a standard ATM cell is inputted to the ATM switching apparatus, the ATM switching apparatus reads out an output side VPI/VCI corresponding to a VPI/VCI in the standard ATM cell (input side VPI/VCI) from the table, and replaces the input side VPI/VCI in the standard ATM cell to the output side VPI/VCI (header conversion). Hereafter, the ATM switching apparatus determines the output path of the standard ATM cell in accordance with the output side VPI/VCI (routing), and transmits the ATM cell from the output path.
FIGS. 20(A), (B) is diagram showing an example of a header conversion apparatus 101. As shown in FIG. 20(A), the header conversion apparatus 101 has a header conversion table 102 storing a new VPI, VCI, CID (an output side VPI, VCI, CID) corresponding to the input side VPI, VCI. When a partial fill cell is inputted to the header conversion apparatus 101, the header conversion apparatus 101 obtains the input side VPI, VCI from the partial fill cell, reads out a new VPI, VCI, CID as data corresponding to the input side VPI, VCI as an address from the header conversion table 102. The new VPI, VCI, CID is stored in a AAL type 2 cell.
As shown in FIG. 20(B), the header conversion apparatus 103 has a header conversion table 104 storing a new VPI, VCI (an output side VPI, VCI) corresponding to the input side VPI, VCI, CID. When a AAL type 2 cell is inputted to the header conversion apparatus 103, the header conversion apparatus 103 obtains the input side VPI, VCI, CID from the AAL type 2 cell, reads out a new VPI, VCI as data corresponding to the input side VPI, VCI, CID as an address from the header conversion table 104. The new VPI, VCI is stored in a partial fill cell.
As shown in FIG. 20(B), the header conversion apparatus has a header conversion table 104 storing a newly VPI, VCI (a output side VPI, VCI) corresponding to the input side VPI, VCI, CID. When a AAL type 2 cell inputed the header conversion apparatus 103, the header conversion apparatus 103 obtains the input side VPI, VCI, CID from the AAL type 2 cell, reads out a newly VPI, VCI as data corresponding to the input side VPI, VCI, CID as a address from the header conversion table 104. The newly VPI, VCI is stored in a partial fill cell.
However, the header conversion table 102 needs 28 bits for an address region (VPI:12 bits, VCI:16 bits), and 36 bits for the data region (VPI:12 bits, VCI:16 bits, CID:8 bits) to one ATM connection. Therefore, when all the patterns capable of being set are stored into the header conversion table 102, the conversion table 102 needs bulky memory capacity. The header conversion table 104 also needs bulky memory capacity. As a result, influence on costs is great.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a short cell multiplexing apparatus which is able to carry out switching for every short cell and to restrain an upscale of hardware.
Like a conventional method, memory volume required for the header conversion table may be reduced by restricting the number of significant digits of the VPI/VCI. However, this requires to add CID (8 bits) to the VPI/VCI, resulting in larger memory volume in the table compared with the conventional method.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a short cell multiplexing apparatus which is able to carrry out switching every a short cell and to restrain an upscale of a hardware.
The present invention employs the following configuration to solve the above described problems. Namely, the short cell multiplexing apparatus of the present invention comprises a first header converting section, a short cell multiplexing section, and a second header converting section. The first header converting section outputs, when a plurality of partial fill cells which are standard cells each storing one short cell are inputted to the first header converting section, the partial fill cells, while converting a virtual path identifier stored in each of the partial fill cells into a value that is a unit of a multiplexing process at the short cell multiplexing section, and converting a virtual connection identifier stored in each of the partial fill cells into a value to be a short cell connection identifier which is to be stored in the short cell at the short cell multiplexing section. The short cell multiplexing section receives a plurality of the partial fill cells outputted from the first header converting section to multiplex a plurality of the short cells which are stored in the partial fill cells for every virtual path identifier stored in each of the partial fill cells, and produces a cell (for example, AAL Type 2 cell) in which virtual channel identifiers of the partial fill cells are stored as the connection identifiers of the short cells and the virtual path identifiers of the partial fill cells storing the short cells are stored as the virtual channel identifiers, and outputs the cell. The second header converting section receives the cell outputted from the short cell multiplexing section and outputs the cell while converting the respective virtual path identifiers and the virtual channel identifiers which are stored in the cell into given values.
The present invention carries out header conversion for the partial fill cells and the AAL Type 2 cells so that the ATM switching apparatus may carry out ATM cell switching for every short cell. Thus, the use of the ATM switching apparatus enables routing for the short cells. In addition, in the header conversion process, no simultaneous VPI, VCI, CID storage is required for the addresses of a table for header conversion. The result reduces memory capacity required for the table to diminish the hardware configuration of the short cell multiplexing apparatus.